Improve Android-change documentation in RandomAccessFile

This CL is a refactoring only and does not change behavior.

Android changes in this file were insufficiently documented.
To minimize the diff vs. upstream OpenJDK8u121-b13, this CL
reverts an Android change in the constructor where a local
variable imode had been replaced with code that directly
writes to this.mode.

The logic in the constructor for computing the mode value
has two Android changes the reason for which is undocumented
and the correctness of which is questioned by a comment.
This CL tweaks the comments to better clarify what Android
changed and that the correctness is unclear but doesn't change
the logic. The correctness question will need to be addressed
by a future CL.

Two exception messages are also different on Android.

The aforementioned changes were all made in libcore commit
8d05e88f57c1ea5543d4012687c70cd64efcada0 (May 2015,
"Use OpenJdk implementations for java.io.* [part 2]").

Bug: 35910877
Test: Treehugger build succeeds.

Change-Id: I353626b3cbc9ec8fa4d7ba26408723547627e78d
1 file changed