Add @Nullable annotation to the parameter of Object.equals() methods.

Those annotations could be inferred by some tools (like Kotlin), but the
https://checkerframework.org/ doesn't check inherited annotations
complaining about all equals() invocations that get nullable argument.

The change was generated by running

find . -name \*.java | xargs sed -i 's/public boolean equals(Object /public boolean equals(@Nullable Object /'

in the frameworks/base directory and by automatically adding and
formatting required imports if needed. No manual edits.

Bug: 170883422
Test: Annotation change only. Should have not impact.
Exempt-From-Owner-Approval: Mechanical change not specific to any component.
Change-Id: I5eedb571c9d78862115dfdc5dae1cf2a35343580