Add history and persistence support to Evaluator.

Bug: 31623549
Bug: 31686717

This version superficially works, but introduces a number of FIXME
comments that should be repaired before shipping. It may not stand
up well to monkey tests.

Add ExpressionDB.java that encapsulates a SQLite database to hold
the calculator history.

Change Evaluator to support simultaneous evaluation of many different
expressions, such as those in the history. In order to avoid UI thread
blocking, the evaluator now loads referenced subexpression on demand,
and multiple expressions can be evaluated simultaneously. We handle
the concurrency somewhat optimistically, arranging to have concurrent
evaluations not step on each other, at the expense of occasionally
throwing away a redundant result.

Change the expression serialization algorithm to make formulas more
compact. Do this now to minimize annoying database format changes.

Persist the saved clipboard and "memory" state.

Add two buttons in landscape mode to allow minimal access to "memory".

Since this involved a substantial rewrite of Evaluator.java, it
includes some drive-by cleanups and minor bug fixes.

Notably:
mMsdIndex was apparently always recomputed, but never actually
saved. Oops. This no doubt added a small amount of overhead.

We updated the decimal conversion size limits, hopefully addressing
b/30200325, possibly at some expense of slightly worse behavior
on old 32-bit devices.

Tests: Tested manually with a bit of added instrumentation.
Ran existing automated tests.

Change-Id: Ifae408d7b4b6cacd19f0e8f5aca146e9c653927e