Improve flakiness of PlatformCompatGating test

The PlatformCompatGating test did not close the ParcelFileDescriptor
opened by UIAutomation.executeShellCommand, which is the likely culprit
of the test throwing a java.io.InterruptedIOException randomly.
The documentation for executeShellCommand requires the users to do the
cleanup themselves.

Test: atest PlatformCompatGating
Bug: 178108878
Change-Id: Iaf4d82d5fe4170931d2c59f2978e545b15e940b0