Implement percent and new inverse functions

Bug: 21493470

Add x^2 10^x and e^x functions, to make the recently added INV key
work as expected.

Implement % functionality.

10^x is essentially just macro expansions for now.

% and x^2 need trivial evaluator support to provide reasonable display

We decided to add evaluator support for exp() as well.

Add corresponding exp() support to BoundedRational and its tests.

Tiny incidental changes for problems uncovered in the process:
Fix bug in tests/README.txt
Evaluate the constant e only once.
Add one more power test along with the exp() test.
Fix proguard.flags so BRTest runs again.

Change-Id: I26cfcaf6d99aeec11387297cc5586e2ddcab6add
11 files changed