Hardcode maximum heap size

[sjchmiela]

Checklist

• [x] I've read the Contribution Guidelines.
• [ ] I've updated the CHANGELOG if necessary.
• [x] I've ensured the unit and smoke tests are still passing - either by running yarn test:unit and yarn test:smoke:[ios|android] or by checking the appropriate CircleCI builds' statuses.
• [x] I've manually tested whether the changes I made work as expected.

Motivation and Context

Builds started to fail with

Error occurred during initialization of VM
Initial heap size set to a larger value than the maximum heap size
Error: ./gradlew exited with non-zero code: 1
    at ChildProcess.completionListener (/app/turtle/node_modules/@expo/xdl/node_modules/@expo/spawn-async/build/spawnAsync.js:52:23)
    at Object.onceWrapper (events.js:418:26)
    at ChildProcess.emit (events.js:311:20)
    at ChildProcess.EventEmitter.emit (domain.js:482:12)
    at maybeClose (internal/child_process.js:1021:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:286:5)
    ...
    at spawnAsync (/app/turtle/node_modules/@expo/xdl/node_modules/@expo/spawn-async/build/spawnAsync.js:17:21)
    at spawnAsyncThrowError (/app/turtle/node_modules/@expo/xdl/build/detach/ExponentTools.js:201:45)
    at buildShellAppAsync (/app/turtle/node_modules/@expo/xdl/build/detach/AndroidShellApp.js:940:11)
    at async Object.createAndroidShellAppAsync (/app/turtle/node_modules/@expo/xdl/build/detach/AndroidShellApp.js:386:5)
    at async runShellAppBuilder (/app/turtle/build/builders/android.js:95:9)
    at async Object.buildAndroid [as android] (/app/turtle/build/builders/android.js:43:28)
    at async build (/app/turtle/build/jobManager.js:181:33)
    at async processJob (/app/turtle/build/jobManager.js:118:32)
    at async Object.doJob (/app/turtle/build/jobManager.js:49:5)
    at async main (/app/turtle/build/server.js:66:13)

Description

Consulted and helped with by @wkozyra95 — removed experimental options that made maximum heap size being set to a low value. Hardcoded proper value.